This invention relates to a method and apparatus for continuously and non-invasively measuring cutaneous blood pressure in a small isolated flesh element. The physiological data thus obtained are related to, but not identical, to blood pressure measurements of a more central arterial circulation, such as that obtained with the conventional auscultatory method of estimating brachial blood pressure.
In particular, the invention finds use as part of a general system for measurement of blood pressure based on repetitive evaluation of the cutaneous pressure fluctuation patterns of minute branches of larger arteries and therefore reflects the arterial blood pressure of the general circulation. This is to be expected since the latter is the source of the former.
The method and apparatus of the present invention enables continuous monitoring of blood pressure patterns over extended periods of time. This is needed in the evaluation of circulatory function and ambulatory monitoring of cardiac function, and is useful for hypertension studies and for obtaining records of circulation in the peripheral systems, particularly of the limbs, fingers and toes.
In the past, various artery occlusion procedures have been used stopping blood flow in radial, brachial, dorsalis pedis, temporal and other arteries to estimate blood pressure, particularly of the central circulatory system. Data thus obtained is by its very nature discontinous.
It has been possible to insert pressure sensing devices and/or catheters temporarily into the arteries of the circulatory system for direct continuous measurements (invasive method of measuring). While intra-arterial catheterization may provide more precise measurements of blood pressure than arterial occlusion devices, the pressure measured is likely to be more related to the central circulation, than the peripheral circulation. Also, the blood pressure measurements and patterns thus obtained are likely to be altered by the traumatic operation of inserting the catheter, by the drug administered so that the catheter can be inserted, and by the presence of a foreign body in the circulatory system.
The principal non-invasive blood pressure measuring device used today is an auscultatory system where a pressure is applied to occlude a major artery, such as the brachial artery. In practice, an inflatable encircling cuff is placed around the arm and inflated to occlude the major artery, e.g., brachial, to prevent flow of blood in the artery. As the pressure in the cuff is slowly lowered, permitting flow of blood in the artery, Korotkoff sounds are heard. The cuff pressure at which the first sound is heard is defined as the systolic pressure. The pressure in the cuff is then lowered further the pressure in the cuff at which the sound fades is defined as the diastolic pressure.
A second occluding cuff technique uses palpation of the pulse rather than auscultation. In this palpatory system, as the occluding cuff pressure is slowly released, arterial pulsations are detected by palpation. The pressure level of the cuff at which the pulsations are first perceived is designated as systolic blood pressure. Diastolic blood pressure cannot be detected by palpation.
Another occluding cuff system uses the maximum and minimum oscillations of arterial blood pressure as referenced to cuff pressure as indications of systolic and diastolic blood pressure, respectively. In addition to being an intermittent, occlusive technique, the measurements thus obtained are likely influenced by the limb volume of the limb around which the cuff is applied.
It can be generally stated that all blood pressure measurements which are based upon arterial occlusion are inherently discontinuous, needing to be repeated, at best, from time to time. Such measurements cannot resolve blood pressure patterns on a beat to beat basis, or show the wave form of the individual beats.
Thus, although the current method of auscultatory measurement of brachial blood pressure is by far the most widely used technique for blood pressure measurement, the technique is relatively imprecise, since the observed values vary from observer to observer and the very act of taking blood pressure itself causes a momentary change in blood pressure. Additionally, since the occlusion itself is known to have physiological and psychological effects, the measurements may be distorted.
A non-invasive, non-occlusive approach to the measurement of blood pressure would have many advantages. Unfortunately, prior techniques for this purpose have been found to have disadvantages. Those directed at measuring arterial pressure by placing a transducer directly over a partially compressed radial or dorsalis pedis artery can, under optimum circumstances, provide accurate records for short periods of time. However, the required counter pressure has to be maintained, e.g. with a pneumatic system, and considerable difficulty is experienced in maintaining constant mechanical coupling between the tissue overlying the artery and pressure on the arterial wall during even the slightest patient motion.
An example of this type of measuring system is disclosed in U.S. Pat. No. 3,880,145 to E. F. Blick, issued Apr. 29, 1975. Blick described a system using a strain gauge to flatten the radial artery at the inside of the wrist. A second sensor is mounted cutaneously alongside but off the artery. The signal from the second sensor was subtracted from that sensor associated with the flattened artery. In practice, the signal from the radial artery sensor contains arterial pulsations as well as "noise". The noise which is measured by the cutaneous transducer is subtraced from the former signal, leaving a measurement of the arterial pulsations alone. Such systems are complex and during patient movement it is very difficult to precisely match the "noise" component arising from both sensors.
In the prior devices discussed above, the majority are directed to the measurment of blood pressure in major arteries Elastic strain gauge techniques which encircle limbs or digits have also been used. In such devices, a finger or toe is encircled with a latex or silastic tube which contains mercury. As the digit volume increases with arterial inflow and decreases with venous outflow, the change in volume can be measured and related to blood pressure. However, the system is occlusive in nature and markedly decreases capillary blood flow. Hence, again it can be used only intermittently since it causes distortion of the physiological data. It is not possible to obtain continuous blood pressure records for hours at a time. It is difficult to calibrate since it is temperature sensitive, and must be calibrated off of the body part.
Hand and thumb plethysmographs are also known which measure changes in volume of entire digits, hands, feet, or limbs, but they are very cumbersome and cannot be used on an active patient, such as one jogging.
A general discussion and review of various previously proposed systems for blood pressure monitoring is given in the book, "The Direct and Indirect Measurement of Blood Pressure," by L. A. Geddes (Year Book Medical Publishers, Chicago, 1970) where a number of blood pressure techniques are outlined (see pages 37, 71, 87 and 96).
It has been demonstrated that there is a hitherto unfulfilled need for a sensitive, continuous, non-invasive, non-occlusive measuring technique for recording blood pressure measurements and beat to beat patterns undistorted and uninterrupted by the measuring system, per se. The method and the apparatus of the present invention enables non-invasive, non-occlusive continuous measurements over extended periods of time. Continuous information of this type is essential for adequate evaluation of cardiac and vascular function. It is of particular importance in the diagnosis and treatment of hypertension, since it provides detailed information concerning the peripheral circulation not available heretofore.